docs: update docs for beginners
QuickStart page was updated with more relevant information.
Key Concepts was added to cover basics for the VictoriaMetrics.
To add a Copy button wrap code snippet with the following element:
```
<div class="with-copy" markdown="1">
<your-code-snippet>
</div>
```
See the changes to `Kubernetes monitoring with VictoriaMetrics Single` for details.
